2.4. Measurement of Lactate Dehydrogenase Levels
The level of lactate dehydrogenase (LDH) released from the cells was measured using an LDH assay kit (A020; Nanjing Jiancheng Biology Engineering Institute, Nanjing, Jiangsu, China). VSMC-derived foam cells were treated with 0, 1, 10, and 100 μg/mL ox-LDL for 24 h. The cell supernatants were collected for analysis. Matrix buffer, coenzyme I, and 2,4-dinitrophenylhydrazine solution were added to the samples, which were then incubated at 37°C for 15 min. The reaction was then terminated by adding an appropriate amount of 0.4 mol/L NaOH solution. The absorbance of the sample was determined at 450 nm using a microplate reader (LabX, Austria).